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Northwest Iowa Crop Update Newsletter
by Todd Vagts
ISU Extension Crops Specialist
Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.

[Home][Special Topics][Weather Data][Subsoil H20][PDF Info] [ISU Extension][IA State University]

Volume 5, Number 04

Northwest IA Crop Update, April 11, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Soil temps much above normal
bullet Forecasted degree-days above normal
bullet Obtain seed germ tests before planting
bullet Optimum corn planting dates nearing
bullet Evaluate Alfalfa stands now
bullet Do not over apply nitrogen
bullet Potential ammonia injury to corn
bullet Prepare to scout for alfalfa weevil
bullet Tracking soybean rust
bullet Soybean rust and organic production

A very nice rain fell across much of western IA Sunday night with more expected through Tuesday.  The precipitation is well received as topsoil moisture has been depleted in many areas with the tillage and warm drying winds the area has received over the last 7 to 10 days.  Mild weather is expected to persist over the next week as forecasted degree-days are much above normal for early April.  Corn planting will most likely begin as soon as soils dry, as the optimum corn planting window (April 20) is just around the corner.  Now is a good time to make the last evaluation on alfalfa stand health and quality, in addition to counting stems, be sure to dig crowns and roots to evaluate whole plant health.   Alfalfa weevils will soon be present as the area approaches the accumulation of 250 degree-days, careful scouting for this pest can prevent 1st crop losses.  Not much new has occurred with the spread of soybean rust (other than the two counties in Florida) except for the beginning of scouting of sentinel plots in the south, with no positive rust finds as of yet.

Weather information
Soil Temperature Area soil temperatures are running much above normal (figure 1) and should continue to stay at least above 50 F in the near term as the extended forecast calls for mild temperatures.  Continue to monitor soil temperatures at this web address: and the three day forecast at this URL:


Growing Degree Day accumulation is shown in Figure 2 and is forecasted through April 17.  The forecasted degree-day accumulation is much above normal for the next week, with 61 degree-days forecasted, well above the average of 23 for the same time period.  Typically, corn will emerge with an accumulation of 110 to 140 degree-days (base 50).  Growing degree days are calculated by the following equation: 

(Daily high temp + Daily low temp)/2 – base temperature. 

The base temperature for corn is 50 degrees F and the maximum temperature is 86 degrees F.  Degree-days required for emergence will vary depending on soil temperature, soil type, planting depth, and soil moisture.


Crop Management
Obtaining and interpreting seed germination tests results:  It is important to understand and interpret the seed germ results and plant the seed according to the seed quality and soil conditions.  When planting into a less than ideal soil environment (cool, wet soils) use high quality seed.  Request seed germ tests for the seed lots you plan to plant from your local seedsman. More information from the ISU Seed lab can be obtained from their web site    For answers to the ISU Seed Lab’s most Frequently Asked Questions, visit their FAQ web site:

Corn planting date  Soil temperatures and field conditions have tempted some to plant corn early while favorable conditions exist.  Yet caution is warranted with such early plantings in IA, as data collected from 1998 – 2000 indicated that optimum corn yields are obtained with corn planted between April 20 and May 5.  Yield depression may occur with corn planted before April 20.  Optimum corn planting rates for IA range from 26,000 to 30, 000 plants per acre (final stand).

To determine an appropriate corn or soybean seeding rate, use the following formula:

Seeding rate = Plant population per acre at harvest/(Seed germination x Expected survival) 

Table 1.  Corn Yield as affected by planting date


Table 2.  Plant density effects on corn yield (1997 – 00)


% Yield

April 10


April 20-May 5


May 10


May 20


June 1


June 10




Final Stand

% Yield












Data by Dr. Dale Farnham, Former ISU Extension State Corn Specialist

Forage/Pasture Management
Evaluate alfalfa stands now
  I believe alfalfa over-wintered fairly well, but now is the time to make the evaluation.  You can count stems, plants and dig crowns to evaluate stand thickness and plant health.  Fifty or more stems per square foot should provide very good forage yields, if there are less than forty stems per square foot then another crop should be considered.  The inside of crowns should be a nice creamy white color, if more than 50% of the crown is discolored or rotten, the plant most likely will not survive the season.  For more information on assessing winter injury in alfalfa, go to this web address at the University of Wisconsin that has a good discussion on winter injury:

Soils and Soil Fertility
Nitrogen fertilizers have been going onto area fields over the last several weeks.  The following table gives the ISU recommendation for Nitrogen rates in Corn.  More information on N Fertility in corn can be obtained from ISU publication PM-1714, found on the web at


Table 1.  Suggested N rates for corn production based on crop rotation.


Nitrogen Rate, lb N/acre

Corn after established alfalfa

0 - 30

Second-year corn after alfalfa

0 - 60

Corn after corn

150 - 200

Corn after soybean

100 - 150

Adapted from Table 1 of ISU publication Pm-1714, Nitrogen Fertilizer Recommendations for Corn in Iowa.

Ammonia Injury to Corn  If you just applied anhydrous ammonia to your cornfield, you may be wondering how long to wait to plant corn after the ammonia application.  If there is good soil separation between the ammonia zone and the seed, planting can be done the same day the ammonia is applied. If the seed is to be placed in the ammonia zone, the longer the waiting period the less potential for root injury. There is no magic number of days to wait.  Ammonia injury causes desiccation of roots and die-back from the root tips. Injury is first noted in the field by uneven emergence, slow growth of some plants, and wilting of plants in dry weather. Root injury is noted by browning of the roots and in severe cases roots will be completely killed and turned black back to the seed. Ammonia injury is detected more frequently in dry weather because roots are slow to develop and destruction of a portion of the root systems limits water uptake.  (Information from the ISU ICM newsletter 4-16-93)

Pest Management
Alfalfa weevil: Weevil scouting in alfalfa should begin when accumulated degree-day (DD) counts reach 250.  Current degree-day counts range from 180 to 200 with 250 forecasted to be reached by April 16.  Alfalfa weevil can be very destructive to first cutting alfalfa. They remove leaf tissue, beginning with the new leaves in the top of the plant, then work down the stem to other leaves. Their feeding reduces forage quality and quantity. Begin scouting on south-facing hillsides. Larvae will hatch there first because these areas warm up quicker than northern hillsides.  For more information on the alfalfa weevil, refer to the April 04 2005 issue of the ICM newsletter or go the ICM newsletter website and search on “alfalfa weevil”.

Weed emergence for many species is not a random event and can be predicted to help in scouting, management and control.  Giant ragweed, lambsquarters, Pennsylvania smartweed and common sunflower are typically the first weeds to emerge and can be found in fields before corn and soybeans are planted.  For more information:

Soybean Rust
Tracking soybean rust  The USDA opened a new web site which will track three elements of soybean rust during the season: 1) Counties with confirmed soybean rust 2) Counties with fungicides being applied to manage soybean rust, and 3) Counties where scouting for soybean rust is recommended.  Be sure to bookmark this web URL: and continue to monitor the site during the spring and summer season.

Sentinel plots have been set up across the U.S. as well as IA (read about sentinel plots in the ISU ICM newsletter found here: with many of the southern U.S. plots emerged, scouted and no rust found.  These locations show up on USDA’s rust tracking page as scouted and not found.  Pay close attention to locations in south TX and Louisiana as spores produced in these locations would most likely move across IA if blown north. 

Spore dispersal Forecast  Sunday’s (April 9) spore dispersal forecast is the first to bring moderate risk to states as far west as Mississippi and Louisiana.  Continue to monitor this web page throughout April and May.

Soybean Rust Manual by Ohio State  A compendium addressing all issues of soybean rust has been produced and is available on the web through Ohio State University, you can access the manual at the URL: or go to the website and order a hard copy.

Have questions about soybean rust and federal crop insurance?  The RMA has posted some commonly asked questions and answers with a PDF file at this web address:

Soybean rust and organic soybean production  Concerned about soybean rust and organic soybeans?  Dr. Kathleen Delate (ISU Extension Organic Specialist) address some of the research that will be occurring with organic systems and soybean rust is the 03/14/2005 edition of the ISU ICM newsletter…read the article at this URL:


Print or view this newsletter in  PDF or Microsoft Word format.

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366

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This page last updated on 04/11/05

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